The present invention relates to an improvement in an automatic transmission equipped with a belt drive continuously-variable speed transmission means, which comprises two variable-pitch pulleys and a belt of a V-shape or a trapezoidal cross section extended between the variable-pitch pulleys and is adapted to vary the rotational speed ratio between the respective shafts carrying the variable-pitch pulleys by varying the radial working position of the belt on the variable-pitch pulleys.
There has been known that a continuously-variable speed transmission means, designated as "a belt drive transmission means" hereinafter, comprises a primary variable-pitch pulley and a secondary variable-pitch pulley mounted on an input shaft connected to a prime mover and an output shaft connected to a driven unit respectively, each variable-pitch pulley comprising a fixed pulley flange fixed to the corresponding shaft, a movable pulley flange mounted axially slidably on the same shaft and a spring or the like means urging the movable pulley flange toward the corresponding fixed pulley flange, and a V-belt extended between the primary and the secondary variable-pitch pulleys.
Recently, a link belt, which is formed by linking a number of metallic blocks, each having a cross section similar to that of a V-belt and inclined side surfaces formed so as to fit to the conical surfaces of the fixed and movable pulley flanges and having slits formed in the inclined side surfaces, with metallic belts by fitting the metallic belts in the slits of the metallic blocks, has been developed. The employment of such a link belt in the belt drive transmission means contributes to enhancing the durability of the belt drive transmission means. Thus, trials have been made to employ the belt drive transmission means as an automatic transmission for a motor vehicle equipped with a high-power mover by increasing the transmission torque of the belt drive transmission means by pressing the movable pulley flanges toward the corresponding fixed pulley flanges with hydraulic servomechanisms.